SPARC support for one.(article implies many customers running sparc on older than 2010 hardware that cannot run 11.4).
"UltraSPARC is a Tier 2 architecture through FreeBSD 12.x. It is no longer supported in FreeBSD 13.0 and later"
"Tier 2 platforms are functional, but less mature FreeBSD platforms. They are not supported by the security officer, release engineering, and port management teams."
Official support for Oracle server software for two.
"Oracle Corporation does not officially support the Oracle database or any other of their products on FreeBSD."
At this point I'd wager the bulk of the Solaris installs out there are for running Oracle server software. If there was to be a migration it would likely be to Oracle linux.
(Last time I touched Solaris was 2006)